High velocity spraying processes based on combustion of oxygen-fuel mixtures (HVOF), air-fuel mixtures (HVAF), and/or plasma jets allow coatings to be sprayed from variety of materials. Such processes may generally produce high velocity gas or plasma jets. High quality coatings can be sprayed at a high level of efficiency when the temperature of the jet is high enough to soften or melt the particles being sprayed and the velocity of the stream of combustion products is high enough to provide the required density and other coating properties. Different materials require different optimum temperatures of the sprayed particles in order to provide an efficient formation of high quality coatings. Higher melting point materials, such as cobalt and/or nickel based alloys, carbides and composite materials, may often require relatively high temperatures in order to soften the particles to a level sufficient to efficiently form high quality coatings.
The efficiency of plasma thermal spray systems, and of coating produced using plasma thermal spray systems, may be effected by a variety of parameters. Properly establishing a plasma jet and maintaining the operating parameters of the plasma jet may, for example, be influenced by the ability to form a stable arc having a consistent attachment to the anode. Similarly, the stability of the arc may also be a function of erosion of the anode and/or erosion of plasma jet profiling or forming unit. Erosion of the anode and/or of the forming unit may change the profile of the plasma cavity. Changes of the interior profile of the plasma cavity may result in changes in the characteristics of the plasma jet produced by the plasma torch. Additionally, the quality of a coating produced by a plasma spray system may be affected by consequential heating of the substrate being coated. For example, excessive heating of the substrate may result in diminished coating characteristics.